Light emitting device, method for manufacturing thereof and electronic appliance

ABSTRACT

An object of the invention is to provide a method for manufacturing a light emitting device capable of reducing deterioration of elements due to electrostatic charge caused in manufacturing the light emitting device. Another object of the invention is to provide a light emitting device in which defects due to the deterioration of elements caused by the electrostatic charge are reduced. The method for manufacturing the light emitting device includes a step of forming a top-gate type transistor for driving a light emitting element. In the step of forming the top-gate type transistor, when processing a semiconductor layer, a first grid-like semiconductor layer extending in rows and columns is formed over a substrate. The plurality of second island-like semiconductor layers are formed between the first semiconductor layer. The plurality of second island-like second semiconductor layers serve as an active layer of the transistor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light emitting device manufactured byusing a large size substrate and a method for manufacturing thereof.

2. Description of the Related Art

A light emitting device utilizing light emission of anelectroluminescent element (a light emitting element) has beenattracting attention as a display device with wide viewing angle and lowpower consumption. In recent years, development for the purpose ofmass-production of such the light emitting devices has been carried out.

One of the problems in the mass-production of the light emitting devicesis the development of a technique for manufacturing the light emittingdevices by using a large size substrate. By using the large sizesubstrate, a large size television and the like can be manufactured. Inaddition, small light emitting devices that are mounted on smallelectronic appliances such as cellular phones can be mass-produced byusing a large size substrate.

However, a large-scale processing apparatus is required in processing alarge size substrate when manufacturing a light emitting device usingthe large size substrate. Therefore, it is difficult to process theentire surface of the substrate under uniform conditions. When using asubstrate made from a material that is easily charged, e.g., glass,charge is easily accumulated in a part of the substrate in plasmaprocessing and so on if the entire surface of the substrate is notprocessed under the uniform conditions. When the accumulated chargemoves through elements during process, a large amount of current flowsthrough a migration path of the charge so that deterioration of theelements is sometimes caused.

In order to reduce the deterioration due to the foregoing electrostaticcharge, various measures, e.g., arrangement of a short ring that isconnected to an input terminal for transmitting signals to a displayportion by a conductive film, have been tried. Additionally, a techniqueof providing a electrostatic charge absorption pattern made from a filmof a wiring layer or an electric capacitance is proposed in JapanesePatent Application Laid-Open No. Hei 6-75246 [Patent document 1] so asto reduce the deterioration of the elements due to the electrostaticcharge.

When using the short ring, however, it is difficult to prevent thedeterioration of the elements due to electrostatic charge caused inprocess up to formation of the short ring. Further, unnecessary portionsafter fabricating elements thereover are caused in the techniquedisclosed in the patent document 1, and hence, the entire surface of thesubstrate cannot be utilized effectively.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method formanufacturing a light emitting device, wherein deterioration of elementsdue to electrostatic charge caused in manufacturing the light emittingdevice can be reduced. Further, it is another object of the invention toprovide a light emitting device with reduced defects that are caused bythe deterioration of the elements due to electrostatic charge.

In an aspect of the invention, a method for manufacturing a lightemitting device includes a step of manufacturing a top-gate typetransistor for driving a light emitting element. In the step ofmanufacturing the transistor, when processing a semiconductor layer, afirst grid-like semiconductor layer extending in rows and columns isformed over a substrate and a plurality of second island-likesemiconductor layers is formed between the first grid-like semiconductorlayer. The second island-like semiconductor layers serve as activelayers of the transistor. The top-gate type transistor is a transistorwhose active layer is formed before the formation of the gate electrodeof the transistor.

In another aspect of the invention, a method for manufacturing a lightemitting device includes a step of manufacturing a transistor fordriving a light emitting element. Here, the transistor is formed bysequentially laminating a semiconductor layer, an insulating layer and aconductive layer. In the step of manufacturing the transistor, whenprocessing a semiconductor layer, a first grid-like semiconductor layerextending in rows and columns is formed over a substrate and a pluralityof second island-like semiconductor layers is formed between the firstgrid-like semiconductor layer. The second island-like semiconductorlayers serve as active layers of the transistor.

In another aspect of the invention, a method for manufacturing a lightemitting device includes a step of manufacturing a top-gate typetransistor for driving a light emitting element. In the step ofmanufacturing the transistor, when processing a semiconductor layer, aplurality of groups including a plurality of first island-likesemiconductor layers is formed over a substrate and a second grid-likesemiconductor layer extending in rows and columns is formed so as tosurround each of the plurality of groups. The first island-likesemiconductor layers serve as active layers of the transistor.

In another aspect of the invention, a method for manufacturing a lightemitting device includes a step of manufacturing a transistor fordriving a light emitting element. Here, the transistor is formed bysequentially laminating a semiconductor layer, an insulating layer and aconductive layer. In the step of manufacturing the transistor, whenprocessing a semiconductor layer, a plurality of groups including aplurality of first island-like semiconductor layers is formed over asubstrate and a second grid-like semiconductor layer extending in rowsand columns is formed so as to surround each of the plurality of groups.The second grid-like semiconductor layer serves as an active layer ofthe transistor.

The above-mentioned methods for manufacturing the light emitting devicesmay further include a step of adding an impurity to the first grid-likesemiconductor layer or the plurality of island-like semiconductorlayers. Note that one or both of n-type and p-type impurities may beadded thereinto.

In another aspect of the invention, a light emitting device includes afirst substrate having an element group that includes a light emittingelement and a transistor and a second substrate attached to the firstsubstrate with a sealing material so as to seal the element group. Inthe first substrate, the element group is covered with an insulatinglayer having an opening, the opening is provided in an upper part of asemiconductor layer that surrounds the element group, and the sealingmaterial is provided so as to fill the opening.

In another aspect of the invention, a light emitting device includes afirst substrate having an element group that includes a light emittingelement and a transistor and a second substrate attached to the firstsubstrate with a sealing material so as to seal the element group. Inthe first substrate, the element group is covered with an insulatinglayer having an opening that is provided in an upper part of asemiconductor layer surrounding the element group such that a conductivelayer overlapping with the semiconductor layer is exposed from theopening, and the sealing material is provided so as to fill the opening.

In another aspect of the invention, a light emitting device includes afirst substrate having an element group that includes a light emittingelement and a transistor and a second substrate attached to the firstsubstrate with a sealing material so as to seal the element group. Inthe first substrate, the element group is surrounded by a semiconductorlayer and covered with an insulating film, a conductive layeroverlapping with the semiconductor layer is exposed from the edge of theinsulating layer, and the sealing material is provided to cover theconductive layer and the edge of the insulating layer.

In each of the above-mentioned light emitting devices of the invention,the semiconductor layer is preferably added with one or both of n-typeand p-type impurities. Preferably, the insulating layer is a flat layer,e.g., a layer made from a self-planarizing substance such as acrylic,siloxane (which is a substance including a skeleton structure formed bysilicon(Si)-oxygen(O) bonds and containing at least hydrogen in itsorganic group) and polylmide, or a layer formed by planarizing a siliconoxide layer or the like by CMP (chemical and mechanical polishing) etc.Further, the sealing material preferably contains a substance with a lowmoisture permeability such as bisphenol A liquid resin, bisphenol Asolid resin, epoxy resin containing bromine, bisphenol F resin,bisphenol AD resin, phenol resin, cresol resin, novolac resin, cyclicaliphatic epoxy resin, epibis epoxy resin, glycidyl ether resin,glycidyl amine resin, heterocyclic epoxy resin and modified epoxy resin.

According to the invention, a technique of manufacturing light emittingdevices capable of reducing defects due to electrostatic charge andutilizing a substrate effectively can be obtained. Furthermore, a goodlight emitting device in which the defects due to the electrostaticcharge are reduced can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are top views explaining a method for manufacturing alight emitting device according to the present invention;

FIGS. 2A to 2D are cross sectional views explaining a method formanufacturing a light emitting device according to the invention;

FIGS. 3A to 3C are cross sectional views explaining a method formanufacturing a light emitting device according to the invention;

FIGS. 4A to 4C are cross sectional views explaining a method formanufacturing a light emitting device according to the invention;

FIGS. 5A to 5C are cross sectional views explaining a method formanufacturing a light emitting device according to the invention;

FIGS. 6A to 6C are equivalent circuit diagrams of pixels included inlight emitting devices according to the invention;

FIG. 7 is a top view of a pixel portion for a light emitting deviceaccording to the invention;

FIG. 8 is a top view of a pixel portion for a light emitting deviceaccording to the invention;

FIG. 9 is a top view of a light emitting device according to theinvention; and

FIG. 10 is an electronic appliance applied with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The embodiment modes according to the present invention will hereinafterbe described. The present invention can be carried out in many differentmodes, and it is easily understood by those who skilled in the art thatembodiment modes and details herein disclosed can be modified in variousways without departing from the purpose and the scope of the invention.The present invention should not be interpreted as being limited to thedescription of the embodiment mode to be given below.

[Embodiment Mode 1]

The present embodiment mode will describe a method for manufacturingplural light emitting devices, wherein the plural light emitting devicesare manufactured over a substrate and then the substrate is cut (i.e.,divided) to be used for each light emitting device.

FIG. 1A is a top view explaining the method for manufacturing the lightemitting devices of the invention, while FIGS. 2A to 2D and FIGS. 3A to3C are cross sectional views explaining the method for manufacturing thelight emitting device of the invention. FIG. 2A is a cross sectionalview taken along a dashed line A–A′ of FIG. 1A.

As shown in FIG. 1A, a first grid-like semiconductor layer 102 extendingin rows and columns is formed over a substrate 101. While forming thefirst grid-like semiconductor layer 102, plural second island-likesemiconductor layers 103 are formed inside of regions 130 surrounded bythe first grid-like semiconductor layer 102, as shown in FIG. 2A. Here,the second semiconductor layers 103 is formed so as to fabricatetransistors. Through the steps as described later, plural light emittingdevices each of which is included in each region 130 surrounded by thefirst semiconductor layer 102 as a unit are formed over a substrate 101.In order to form a pixel portion 131 and driver circuit portions 132 aand 132 b within each region 130, respective portions are schematicallyillustrated in the cross sectional views of FIGS. 2A to 2D and FIGS. 3Ato 3C.

Although a material for the substrate 101 is not particularly limitedhere, a substrate made from glass, quartz and the like can be used. Aninsulating layer 104 covering the substrate 101 may be formed on thesubstrate 101. The insulating layer 104 may include a single layer orplural layers. Note that impurities from the substrate 101 can beprevented from dispersing into transistors that will be formed later byproviding a silicon nitride layer (which may contain several % oxygen)in the insulating layer 104.

After forming a semiconductor layer covering the substrate 101, thesemiconductor layer may be processed by etching to form the firstsemiconductor layer 102 and the second semiconductor layers 103. Thesemiconductor layer is not particularly limited here, and may be formedby using silicon and the like. Further, the crystallinity of thesemiconductor layer is not particularly limited, and a semiconductorlayer containing a crystalline component can be employed.

Next, an insulating layer 105 is formed to cover the first semiconductorlayer 102 and the second semiconductor layers 103. The insulating layer105 may include either a single layer or plural layers, and, forexample, can comprise silicon oxide or silicon nitride.

In order to control the threshold value of the transistors, an n-type ora p-type impurity may be added to the second semiconductor layers 103before or after forming the insulating layer 105. For example,phosphorus and the like may be used as the n-type impurity while boronand the like can be used as the p-type impurity. Further, an impuritymay also be added to the first semiconductor layer 102 when adding theimpurity to the second semiconductor layers 103.

Subsequently, conductive layers 106 serving as gate electrodes areformed over the insulating layer 105 at portions where the secondsemiconductor layers 103 and the insulating layer 105 are overlappedwith one another. Concretely, after forming a conductive layer to coverthe insulating film 105, the conductive layer may be processed byetching to form the conductive layers 106. At this moment, the entiresurface of the substrate is maintained at an almost constant potentialby providing the first semiconductor layer 102. Therefore, the potentialdeviation is hardly caused in the surface of the substrate when usingetching with use of plasma excitation such as dry etching, and hence,the elements are hardly damaged by electrostatic charge. The conductivelayers 106 may include a single layer or plural layers. For example, aconductive layer that is well-adhered to the insulating layer 105 may beformed on the insulating layer 105, and a conductive layer with lowresistivity may be laminated thereon. The shape of the conductive layers106 is not particularly limited, and for instance, the sidewalls of therespective conductive layers 106 may have a sloping shape.

Next, an impurity is added to the second semiconductor layers 103 byutilizing the conductive layers 106 as masks. At this moment, animpurity imparting an n-type conductivity such as phosphorus may bedoped to form an n-type transistor. Alternatively, an impurity impartinga p-type conductivity such as boron may be added to form a p-typetransistor. When adding the n-type impurity, the semiconductor layersthat will become constituent elements of p-type transistors may beprotected by utilizing masks made from resist and the like so as not tobe doped with the n-type impurity. Similarly, when adding the p-typeimpurity, the semiconductor layers that will become constituent elementsof n-type transistors may be protected by utilizing masks made fromresist and the like so as not to be doped with the p-type impurity.Alternatively, for example, after adding the n-type impurity to the allsemiconductor layers formed over the substrate 101 without using masksmade from resist and the like, the p-type impurity may be added to thesemiconductor layers that will be the constituent elements of the p-typetransistors by adjusting the amount of the p-type impurity so as tocounteract the n-type conductivity with the p-type conductivity.

The method for adding the impurity is not particularly limited here. Forinstance, doping and the like can be employed. The impurity may also beadded to the first semiconductor layer 102 together with the secondsemiconductor layers 103. By adding the impurity to the firstsemiconductor layer 102, the effect of maintaining the surface of thesubstrate 101 at a constant potential can further be improved.

According to the above-mentioned steps, the transistors 121 a, 121 b,121 c and 121 d formed by laminating the semiconductor layers, theinsulating layer and the conductive layers can be manufactured. Thetransistors 121 a and 121 b are, herein, included in the pixel portion131 and each connected to a light emitting element. The transistor 121 cis included in the driver circuit portion 132 a whereas the transistor121 d is included in the driver circuit portion 132 b. Further, thepixel portion 131 and the driver circuit portions 132 a and 132 b mayinclude another transistors other than the transistors 121 a, 121 b, 121c and 121 d. A structure of the transistors 121 a, 121 b, 121 c and 121d is not particularly limited, and they may have any of a single drainstructure, an LDD structure, another LDD structure in which a LDD and aconductive layer functioning as a gate electrode are overlapped witheach other, and the like, respectively. When the transistors aremanufactured with the first semiconductor layer 102 formed over thesubstrate, the steps of manufacturing the transistors 121 a, 121 b, 121c and 121 d are not particularly limited. Therefore, the steps ofmanufacturing the transistors can appropriately be determined so as toform the transistors 121 a, 121 b, 121 c and 121 d with a predeterminedstructure. Furthermore, all the transistors formed in a light emittingdevice are not necessary to have a same structure, and the structures ofthe transistors may be varied separately depending on an intendedpurpose of each transistor.

Next, an insulating layer 107 is formed to cover the conductive layers106 and the like. The insulating layer 107 may include a single layer orplural layers. In this embodiment mode, the insulating layer 107includes an insulating layer 107 a and an insulating layer 107 b. Afterforming the insulating layer 107 a, the insulating layer 107 a issubjected to heat treatment and then the insulating layer 107 b isformed to cover the insulating layer 107 a. Although the insulatinglayer 107 a is not particularly limited, it is preferably made from aheat resistant substance that can withstand a temperature of 350° C. ormore, e.g., an inorganic material such as silicon oxide, siliconnitride, silicon nitride containing several % of oxygen, and siliconoxide containing several % of nitrogen. Although the insulating layer107 b is not particularly limited, it is preferably formed of a layerthat is made from a self-planarizing substance such as acrylic, siloxaneand polyimide or a layer formed by planarizing a silicon oxide layer orthe like by CMP (chemical and mechanical polishing) etc. Further,although the process conditions of the heat treatment are notparticularly limited, it is preferable that the heat treatment isperformed at a temperature of 350 to 600° C. under an atmosphere filledwith a gas such as nitrogen and hydrogen. Furthermore, the timing of theheat treatment is not particularly limited, and the heat treatment maybe carried out after forming the insulating layer 107 a that is a firstlayer, after forming the insulating layer 107 b that is a second layeror both of after forming the insulating layers 107 a and 107 b.

Next, contact holes that reach the second semiconductor layers 103through the insulating layer 107 are formed. The contact holes may beformed by etching the insulating layer 107. In this case, either dryetching or wet etching can be employed as the etching. Also, acombination of the dry etching and wet etching may be employed. Forinstance, after processing the insulating layer 107 b by dry etching,the insulating layer 107 a may be processed by wet etching to form thecontact holes. Alternatively, both the insulating layers 107 a and 107 bmay be processed by dry etching to form the contact holes.

When forming the contact holes, the entire surface of the substrate 101is maintained at an almost constant potential since the firstsemiconductor layer 102 is provided. Accordingly, when forming thecontact holes by etching with use of plasma excitation such as dryetching, the potential deviation is hardly caused within the surface ofthe substrate, and hence, the elements are hardly damaged by theelectrostatic charge.

Next, wirings 108 and 109 and the like are formed. Concretely, afterforming a conductive layer, the conductive layer may be processed byetching to form the wirings 108 and 109. Note that a material for theconductive layer is not particularly limited, and the conductive layermay include a single layer or plural layers. Preferably, the conductivelayer is formed so as to include a layer made from a substance with lowresistivity such as aluminum. When the wirings 108 and 109 have alamination structure formed by sandwiching an aluminum layer betweentitanium nitride layers, it is possible to prevent the aluminum layer ofeach wiring from being in contact with the second semiconductor layers103 in positions where the wirings and the second semiconductor layersare connected to one another. Further, it is also possible to preventthe aluminum layer from being corroded when using an acidic solution forforming an electrode of a light emitting element later. In the casewhere the wirings 108 and the first semiconductor layer 102 areoverlapped and in contact with each other as shown in this embodimentmode, the first semiconductor layer 102 may also be etched in the stepof etching the wirings 109. Note that the wirings 109 serve to supplycurrent to light emitting elements. The upper part of the firstsemiconductor layer 102 can be utilized efficiently as a region forleading the wirings 109 as shown in the embodiment mode. Note that thewirings 108 and the first semiconductor layer 102 are not necessarily tobe in contact with one another as well as the present embodiment mode,and an insulating layer may be provided therebetween. The wirings 108serve to transmit signals to the respective transistors within theregion 130 surrounded by the first semiconductor layer 102. The wirings108 are connected to the second semiconductor layers 103 through thecontact holes that are previously formed.

When the insulating layer 107 is made from a substance with a highmoisture permeability, a conductive layer for covering sides of theinsulating layer 107 is preferably formed along with the formation ofthe wirings 108 and 109. This can prevent ingress of moisture into thelight emitting element from the outside of the light emitting devicethrough the insulating layer 107.

An electrodes 110 of the light emitting element is next formed. In thiscase, the electrode 110 of the light emitting element is partlyoverlapped with at least the wirings 109 so that the electrodes 110 ofthe light emitting element can be electrically connected to the wirings109. The electrodes 110 of the light emitting element is notparticularly limited. For example, after forming a conductive layer byusing a conductive material that can transmit visible light, theconductive layer can be processed by etching to form the electrodes 110.Although the conductive material that can transmit visible light is notparticularly limited, indium tin oxide (ITO), ITO containing siliconoxide, IZO (indium zinc oxide) formed by mixing 2 to 20% zinc oxide(ZnO) and indium oxide and the like can be employed. Either wet etchingor dry etching can be used as the etching. For instance, a weak acidicsolution can be used when etching the ITO and the like. The electrode110 can also be formed by using aluminum and the like as substitute forthe conductive material that can transmit visible light. Note that thealuminum may include alkali metal (such as lithium (Li)), alkali earthmetal or magnesium (Mg).

A partition wall layer 111 with an opening is next formed. The partitionwall layer 111 preferably has a shape with a radius of curvature that iscontinuously varied at side portions. Additionally, the partition walllayer 111 is formed such that the electrodes 110 of the light emittingelement are exposed from the opening. A substance of the partition walllayer 111 is not particularly limited. For example, acrylic, polyimide,siloxane (which is a substance having a skeleton structure formed bysilicon(Si)-oxygen(O) bonds and containing at least hydrogen in itsorganic group), resist and the like can be used. Photosensitive acrylic,polyimide and resist may be used here. Note that the sidewalls of theinsulating layer 107 is not necessary to be covered with the partitionwall layer 111.

A light emitting layer 112 is next formed to cover the electrodes 110 ofthe light emitting element. The light emitting layer 112 is notparticularly limited as far as a layer containing a light emittingsubstance. For instance, the light emitting layer 112 may include asingle layer made from a substance with a superior light emittingproperty and an excellent carrier transporting property, or a singlelayer or plural layers containing a substance with a superior lightemitting property and a substance with an excellent carrier transportingproperty. A substance for forming the light emitting layer 112 is notparticularly limited, and one or both of organic and inorganicsubstances can be employed.

An electrode 113 of the light emitting element is next formed. Theelectrode 113 of the light emitting element is not particularly limited,and it can be formed of aluminum or the above-mentioned conductivesubstance that can transmit visible light. Note that the aluminum mayinclude alkali metal such as lithium (Li) and magnesium or alkali earthmetal.

Preferably, the thicknesses, materials, lamination structures and thelike of the electrodes 110 and 113 for the light emitting element arecontrolled such that at least one of the electrodes can transmit visiblelight.

According to the foregoing steps, a light emitting element 114 includingthe light emitting layer 112 sandwiched between the electrodes 110 and113 of the light emitting element can be manufactured.

After forming the light emitting element 114, a protective layer 115 maybe provided so as to prevent ingress of moisture and the like into thelight emitting element. The protective layer 115 may include a singlelayer or plural layers and can be formed of silicon nitride or the like.

Through the above-mentioned steps, a plurality of light emitting deviceseach of which includes the pixel portion 131, the driver circuitportions 132 a, 132 b and 133, the wirings 108 formed in the peripheryof the pixel portion and the driver circuit portions, a connectionterminal 134 and the like can be manufactured over the substrate 101 asshown in FIG. 1B. In each pixel portion 131, plural pixels including thetransistors and the light emitting elements are aligned in rows andcolumns. Note that the first semiconductor layer 102 is illustrated inFIG. 1B so as to show a positional relation between constituent elements(the pixel portion 131, the driver circuit portions 132 a, 132 b and133, the wirings 108 provided in the periphery of the pixel portion andthe driver circuit portions, the connection terminal 134 and the like)included in each light emitting device and the first semiconductor layer102. Although one pixel including two transistors and one light emittingelement is illustrated in FIG. 1B, the pixel structure is not limitedthereto.

In the above-mentioned method for manufacturing the light emittingdevices, a short ring may be utilized so as to reduce damage to theelements due to the electrostatic charge, which may be caused afterforming the wirings 108 and 109.

Next, the substrate 101 is cut (i.e. divided) into each light emittingdevice. In this case, a layer is preferably not laminated over cuttingsections along which the substrate 101 is divided into each lightemitting device. In particular, it is preferable that a conductivelayer, or a layer made from an organic material is not laminated overthe cutting sections.

The divided substrates 101 and substrates 140 are attached to each otherwith a sealing material 141 so that light emitting layers are sealedtherebetween. At this moment, the wirings 108 covering the insulatinglayer 107 is preferably covered with the sealing material 141. Further,the sealing material 141 preferably contains a substance with a lowmoisture permeability such as bisphenol A liquid resin, bisphenol Asolid resin, epoxy resin containing bromine, bisphenol F resin,bisphenol AD resin, phenol resin, cresol resin, novolac resin, cyclicaliphatic epoxy resin, epibis epoxy resin, glycidyl ether resin,glycidyl amine resin, heterocyclic epoxy resin and modified epoxy resin.This can inhibit ingress of moisture into the light emitting elementthrough the insulating layer 107 when the insulating layer 107 or thepartition wall layer 111 is made from a substance with a high moisturepermeability. Each of the interior of the light emitting devices afterthe sealing step, i.e., the inner portion surrounded by the substrate101, the substrate 140 and the sealing material 141 may be filled withan inert gas such as nitrogen or a resin material with a low moisturepermeability etc. Or, the inner portion interior may be evacuated.Alternatively, a hygroscopic substance may be fixed in the interior ofthe light emitting device after the sealing step to absorb moistureintruding into the interior thereof so that deterioration of the lightemitting element due to moisture or the like can be prevented. Thehygroscopic substance is not particularly limited. For instance, calciumoxide can be employed. Also, the method for fixing the hygroscopicsubstance is not particularly limited. For example, after providing adepression portion in a part of the substrate 140 and filling asubstance that contains a granular calcium oxide and a fixing agent inthe depression portion, the substance is cured so as to be fixed to theinterior of the light emitting device. Note that the fixing agent isalso not particularly limited, and for example, ester acrylate or thelike can be employed. Additionally, the substrate 140 is notparticularly limited, and a substrate made of glass, quartz, plastic orthe like can be used.

Each of the light emitting devices manufactured above has the firstgrid-like semiconductor layer 102 extending in rows and columns over thesubstrate 101 so as to maintain the substrate 101 at a constantpotential during the formation of the transistors 121 a, 121 b, 121 cand 121 d and the wirings 108 and 109. Therefore, the deterioration ofthe elements due to the electrostatic charge, which is easily caused inthe process utilizing plasma excitation and the like, can be suppressedin the present light emitting devices. The present invention isextremely effective in the case of manufacturing a plurality of lightemitting devices from a large size substrate with an area of 600 mm×720mm or more.

[Embodiment Mode 2]

The method for manufacturing the light emitting devices as described inEmbodiment Mode 1 and another mode of the present invention will bedescribed in the present embodiment mode with reference to FIGS. 4A to4C and FIGS. 5A to 5C.

A first grid-like semiconductor layer 202 extending in rows and columnsis formed over a substrate 201 as well as the first semiconductor layer102 as shown in Embodiment Mode 1. While forming the first semiconductorlayer 202, plural second island-like semiconductor layers 203 are formedinside of each region 230 surrounded by the first semiconductor layer202 in the same manner as the second island-like semiconductor layers103 as shown in Embodiment Mode 1. The second semiconductor layers 203are formed to fabricate transistors. Through steps as described later,plural light emitting devices each of which is included in one region230 surrounded by the first semiconductor layer 202 as a unit are formedover the substrate 201. Note that respective portions are schematicallyillustrated in the cross sectional views of FIGS. 4A to 4C and FIGS. 5Ato 5C so as to manufacture a pixel portion 231, driver circuit portions232 a and 232 b and the like inside each of the region 230.

The substrate 201 is not particularly limited, and the same materialused for the substrate 101 as explained in Embodiment Mode 1 can beemployed here. An insulating layer 204 covering the substrate 201 may beformed on the substrate 201. Similarly, the insulating layer 204 is notparticularly limited, the same material used for the insulating layer104 of Embodiment Mode 1 may be used.

After forming a semiconductor layer covering the substrate 201, thesemiconductor layer may be processed by etching to form the firstsemiconductor layer 202 and the second semiconductor layers 203. Thesemiconductor layer is not particularly limited, and silicon and thelike can be used. Also, the crystallinity of the semiconductor layer isnot particularly limited, and a semiconductor layer containing acrystalline component can be used.

An insulating layer 205 is next formed to cover the first semiconductorlayer 202 and the second semiconductor layers 203. The insulating layer205 is not particularly limited, and it may be formed in the same manneras the insulating layer 105 shown in Embodiment Mode 1.

In order to control the threshold value of the transistors, an n-type ora p-type impurity may be added to the second semiconductor layers 203before or after forming the insulating layer 205. For example,phosphorus and the like may be used as the n-type impurity while boronand the like can be used as the p-type impurity. Further, an impuritymay also be doped into the first semiconductor layer 202 when adding theimpurity to the second semiconductor layers 203.

Subsequently, conductive layers 206 that serves as gate electrodes areformed over the insulating layer 205 at portions where the secondsemiconductor layers 203 and the insulating layer 205 are overlappedwith one another. Concretely, after forming a conductive layer to coverthe insulating film 205, the conductive layer may be processed byetching to form the conductive layers 206. At this moment, the entiresurface of the substrate 201 is maintained at an almost constantpotential by providing the first semiconductor layer 202. Therefore, thepotential deviation is hardly caused in the surface of the substratewhen using etching with use of plasma excitation such as dry etching;and hence, the elements are hardly damaged by electrostatic charge. Theconductive layers 206 may, herein, include either a single layer orplural layers. For example, a conductive layer that is well-adhered tothe insulating layer 205 may be formed so as to be in contact with theinsulating layer 205, and a conductive layer with low resistivity may belaminated thereon. The shape of the conductive layers 206 is notparticularly limited, and for instance, the sidewalls of the conductivelayers 206 may have a sloping shape.

Next, an impurity is added to the second semiconductor layers 203 byutilizing the conductive layers 206 as masks. In this case, an impurityimparting an n-type conductivity such as phosphorus may be added to forman n-type transistor. Alternatively, an impurity imparting a p-typeconductivity such as boron may be added to form a p-type transistor.When adding the n-type impurity, the semiconductor layers that willbecome constituent elements of p-type transistors may be protected byusing masks made from resist and the like so as not to be doped with then-type impurity. Similarly, when adding the p-type impurity, thesemiconductor layers that will become constituent elements of n-typetransistors may be protected by using masks made from resist and thelike so as not to be doped with the p-type impurity. Alternatively, forexample, after doping the n-type impurity to the all semiconductorlayers formed over the substrate 201 without using resist masks and soon, the p-type impurity may be added to the semiconductor layers thatwill be the constituent elements of the p-type transistors by adjustingthe amount of the p-type impurity so as to counteract the n-typeconductivity with the p-type conductivity.

The method for adding the impurity is not particularly limited. Forexample, doping and the like can be used. The impurity may also be addedto the first semiconductor layer 202 along with the second semiconductorlayers 203. By adding the impurity to the first semiconductor layer 202,the effect of maintaining the substrate 201 at a constant potential canfurther be improved.

According to the above-mentioned steps, transistors 221 a, 221 b, 221 cand 221 d formed by laminating the semiconductor layers, the insulatinglayer and the conductive layers can be manufactured. The transistors 221a and 221 b are, herein, included in the pixel portion 231 and each willbe connected to a light emitting element. The transistor 221 c isincluded in the driver circuit portion 232 a whereas the transistor 221d is included in the driver circuit portion 232 b. Further, the pixelportion 231 and the driver circuit portions 232 a and 232 b may includeanother transistors other than the transistors 221 a, 221 b, 221 c and221 d. A structure of the transistors 221 a, 221 b, 221 c and 221 d isnot particularly limited, and they can have any of a single drainstructure, an LDD structure, another LDD structure in which a LDD and aconductive layer functioning as a gate electrode are overlapped witheach other, and the like, respectively. When the transistors aremanufactured with the first semiconductor layers 202 formed over thesubstrate, the step of manufacturing the transistors 221 a, 221 b, 221 cand 221 d is not particularly limited. Therefore, the step ofmanufacturing the transistors may appropriately be determined so as toform the transistors 221 a, 221 b, 221 c and 221 d with a predeterminedstructure. Furthermore, the all transistors formed in a light emittingdevice are not necessary to have a same structure, and structures of thetransistors may be varied separately depending on the intended purposeof the transistors.

Next, an insulating layer 207 is formed to cover the conductive layers206 and the like. The insulating layer 207 may include a single layer orplural layers. In this embodiment mode, the insulating layer 207includes an insulating layer 207 a and an insulating layer 207 b.Concretely, after forming the insulating layer 207 a, the insulatinglayer 207 b is further formed to cover the insulating layer 207 a, andthen the both insulating layers are subjected to heat treatment.Although the insulating layers 207 a and 207 b are not particularlylimited, they are preferably made from heat resistant substrates thatcan withstand a temperature of 350° C. or more, e.g., an inorganicmaterial such as silicon oxide, silicon nitride, silicon nitridecontaining several % of oxygen and silicon oxide containing several % ofnitrogen. Although the process conditions of the heat treatment are notparticularly limited, it is preferable that the heat treatment beperformed at a temperature of 350 to 600° C. under an atmosphere filledwith a gas such as nitrogen and hydrogen. Furthermore, the timing of theheat treatment is not particularly limited, and the heat treatment maybe carried out after forming the insulating layer 207 a that is a firstlayer, after forming the insulating layer 207 b that is a second layeror both of after forming the insulating layers 207 a and 207 b.

Next, contact holes that reach the second semiconductor layers 202through the insulating layer 207 are formed. The contact holes may beformed by etching the insulating layer 207. In this case, either dryetching or wet etching can be employed as the etching. Also, acombination of the dry etching and wet etching may be employed. Forexample, after the insulating layer 207 b is dry-etched, the insulatinglayer 207 a is wet-etched to form the contact holes. Alternatively, boththe insulating layers 207 a and 207 b may be dry-etched to form thecontact holes.

When forming the contact holes, the entire surface of the substrate 201is maintained at an almost constant potential since the firstsemiconductor layer 202 is provided. Accordingly, when forming thecontact holes by etching with use of plasma excitation such as dryetching, the potential deviation is hardly caused within the substrate,and hence, the elements are hardly damaged by the electrostatic charge.

Next, wirings 208 and 209 and the like are formed. After forming aconductive layer, the conductive layer may be processed by etching toform the wirings 208 and 209. Alternatively, after forming theconductive layer, heat treatment may be carried out and then sinteringmay further be performed. Note that a material for the conductive layeris not particularly limited, and the conductive layer may include asinge layer or plural layers. Preferably, the conductive layer is formedso as to include a layer made from a substance with low resistivity suchas aluminum. When the wirings 208 and 209 have a lamination structureformed by sandwiching an aluminum layer between titanium nitride layersor the like, it is possible to prevent the aluminum layer from being incontact with the second semiconductor layers 203 in positions where thewirings and the second semiconductor layers are overlapped with oneanother. Further, the aluminum layer can be prevented from beingcorroded when using an acidic solution for forming an electrode of alight emitting element later. Note that the wirings 208 serve to supplycurrent to a light emitting element. The wirings 208 and the firstsemiconductor layer 202 are overlapped with each other. Accordingly, theupper part of the first semiconductor layer 202 can effectively beutilized as a region for leading the wirings 208. The wirings 209 serveto transmit signals to the respective transistors within the region 230surrounded by the first semiconductor layer 202. The wirings 209 areconnected to the second semiconductor layers 203 through the contactholes that are previously formed.

An insulating layer 210 is next formed to cover the wirings 209.Although the insulating layer 210 is not particularly limited, it ispreferably formed by a layer made from a self-planarizing substance suchas acrylic, siloxane and polyimide, or a layer formed by planarizing asilicon oxide layer or the like by CMP (chemical and mechanicalpolishing) etc. When the insulating layer 210 is made from siloxane,heat treatment for baking the insulating layer 210 may be carried out.

Contact holes that reach the wirings 209 through the insulating layer210 are next formed. While forming the contact holes, openings areformed such that the wirings 208 are partly exposed. The contact holesand the like may be formed by etching the insulating layer 210. In thiscase, either dry etching or wet etching may be employed. Alternatively,a combination thereof may also be employed.

When forming the contact holes, the entire surface of the substrate 201is maintained at an almost constant potential since the firstsemiconductor layer 202 is provided. Accordingly, when forming thecontact holes by etching with use of plasma excitation such as dryetching, potential deviation is hardly caused within the substrate, andhence, the elements are hardly damaged by the electrostatic charge.

Subsequently, electrodes 211 of the light emitting elements that reachesthe wirings 209 via the contact holes passing through the insulatinglayer 210 are formed. The electrodes 211 of the light emitting elementsare not particularly limited. For example, after forming a conductivelayer by using a conductive material that can transmit visible light,the conductive layer is processed by etching to form the electrodes 211.Although the conductive material that can transmit visible light is notparticularly limited, indium tin oxide (ITO), ITO containing siliconoxide, IZO (indium zinc oxide) formed by mixing 2 to 20% zinc oxide andindium oxide and the like can be used. Either wet etching or dry etchingcan be used as the etching. For instance, a weak acidic solution can beused when etching the ITO and the like. The electrodes 211 can also beformed by using aluminum and the like as substitute for the conductivematerial that can transmit visible light. Note that the aluminum mayinclude alkali metal such as lithium (Li) and magnesium or alkali earthmetal.

A partition wall layer 212 with openings is next formed. The partitionwall layer 212 preferably has a shape with a radius of curvature that iscontinuously varied at side portions. Further, the partition wall layer212 is formed such that the electrodes 211 of the light emitting elementare exposed from the openings. A substance of the partition wall layer212 is not particularly limited. For example, acrylic, polyimide,siloxane (which is a substance having a skeleton structure formed bysilicon(Si)-oxygen(O) bonds and containing at least hydrogen in itsorganic group), resist or the like can be employed. Photosensitiveacrylic, polyimide and resist may be used here. Note that the sides ofthe insulating layer 210 may be covered with the partition wall layer212 as well as FIG. 3B.

A light emitting layer 213 is formed to cover the electrodes 211 of thelight emitting element. The light emitting layer 213 is not particularlylimited, and the same material used for the light emitting layer 112 asexplained in Embodiment Mode 1 can be employed.

An electrode 214 of the light emitting element is next formed. Theelectrode 214 of the light emitting element is not particularly limited,and can be formed of aluminum or the above-described conductive materialthat can transmit visible light. Note that aluminum may include thealkali metal (such as lithium (Li)), alkali earth metal or magnesium(Mg).

Preferably, the thicknesses, materials, lamination structures and thelike of the electrodes 211 and 214 for the light emitting element arecontrolled so that at least one of the electrodes 211 and 214 cantransmit visible light.

According to the above-mentioned steps, a light emitting element 215including the light emitting layer 213 sandwiched between the electrodes211 and 214 of the light emitting element can be manufactured.

After forming the light emitting element 215, a protective layer 216 maybe provided so as to prevent ingress of moisture and the like into thelight emitting element. The protective layer 216 may include a singlelayer or plural layers and can be formed of silicon nitride or the like.

Through the above-described steps, a plurality of light emitting deviceseach of which includes the pixel portion 231, the driver circuitportions 232 a and 232 b the wirings 208 formed in the periphery of thepixel portion and the driver circuit portions, a connection terminal andthe like can be manufactured over the substrate 201. In each pixelportion 231, plural pixels including transistors and light emittingelements are aligned in rows and columns. The positional relationbetween constituent elements (the pixel portion 231, the driver circuitportions 232 a and 232 b, the wirings 208 provided in the periphery ofthe pixel portion and the driver circuit portions, the connectionterminal and the like) included in each light emitting device and thefirst semiconductor layer 202 is identical to the one as shown in FIG.1B of Embodiment Mode 1. Note that the configuration of the lightemitting devices is not limited to that of FIG. 1B.

The substrate 201 is cut (i.e. divided) into each light emitting device.In this case, a layer is preferably not laminated over cutting sectionsalong which the substrate 201 is divided into the respective lightemitting devices. In particular, it is preferable that a conductivelayer, or a layer made from an organic material is not laminated overthe sections.

The divided substrates 201 and substrates 240 are attached to each otherwith a sealing material 241 so that respective light emitting layers areencapsulated therebetween. At this moment, the openings formed in theinsulating layers 210 are preferably filled with the sealing material241. Further, the sealing material 241 preferably contains a substancewith a low moisture permeability such as bisphenol A liquid resin,bisphenol A solid resin, epoxy resin containing bromine, bisphenol Fresin, bisphenol AD resin, phenol resin, cresol resin, novolac resin,cyclic aliphatic epoxy resin, epibis epoxy resin, glycidyl ether resin,glycidyl amine resin, heterocyclic epoxy resin and modified epoxy resin.This can inhibit ingress of moisture into the light emitting element 215through the insulating layer 210 when the insulating layer 210 or thepartition wall layer 212 is made from a substance with a high moisturepermeability. The interior of the light emitting device after thesealing step, i.e., the inner portion surrounded by the substrate 201,the substrate 240 and the sealing material 241 may be filled with aninert gas such as nitrogen, a resin material with a low moisturepermeability or the like. Or, the inner portion may be evacuated.Alternatively, a hygroscopic substance may be fixed in the interior ofthe light emitting device after the sealing step to absorb moisture andthe like intruding into the interior thereof so that deterioration ofthe light emitting element due to moisture and the like can beprevented. The hygroscopic substance is not particularly limited. Forinstance, calcium oxide can be employed. Also, the method for fixing thehygroscopic substance is not particularly limited. For example, afterproviding a depression portion in a part of the substrate 240 andfilling a substance that contains a granular calcium oxide and a fixingagent in the depression portion, the substance is cured so as to befixed to the interior of the light emitting device. Note that the fixingagent is also not particularly limited, and for example, ester acrylateor the like can be employed. Additionally, the substrate 240 is notparticularly limited, and a substrate made of glass, quartz, plastic orthe like can be used.

Each of the light emitting devices manufactured above includes the firstgrid-like semiconductor layer 202 extending in rows and columns over thesubstrate 201 so as to maintain the surface of the substrate 201 at aconstant potential during the fabrication of the light emitting devices.Therefore, the deterioration of the elements due to the electrostaticcharge, which is easily caused in the process utilizing plasmaexcitation, can be reduced in the present light emitting devices. Thepresent invention is extremely effective in the case where a large sizesubstrate with an area of 600 mm×720 mm or more is employed so as toform plural light emitting devices by using one substrate.

[Embodiment Mode 3]

Light emitting devices manufactured according to the present inventionas shown in Embodiment Modes 1 and 2 will be described in the presentembodiment mode.

This embodiment mode will describe modes of the light emitting devicesformed according to the present invention. Note that configurations ofthe light emitting devices according to the invention and substances ofthe light emitting devices are not limited to the present embodimentmode.

In the light emitting devices as shown in FIGS. 3C and 5C, the lightemitting layer 112 that is a constituent element of the light emittingelement 114 and the light emitting layer 213 that is a constituentelement of the light emitting element 215 include plural layers,respectively. The plural layers are formed by combining layers made fromsubstances selected from substances with excellent carrier transportingproperties and excellent carrier injecting properties. The plural layerspartly contain a substance with an excellent light emitting property.The substance included in the light emitting layers 112 and 213 may beeither an organic substance or an inorganic substance. In the case ofthe organic substance, it may be either a low molecular weight organicsubstance or a high molecular weight organic material.

With respect to light emitting substances, the following substances canbe used: 4-dicyanomethylene-2-methyl-6-(1,1,7,7-tetramethyl-julolidy1-9-enyl)-4H-pyran (abbreviation: DCJT);4-dicyanomethylene-2-t-butyl-6-(1,1,7,7-tetramethyl-julolidyl-9-enyl)-4H-pyran (abbreviation: DPA); periflanthene;2,5-dicyano-1,4-bis(10-methoxy-1,1,7,7-tetramethyl-julolidy1-9-enyl)benzene; N,N′-dimethylquinacridone (abbreviation: DMQd);coumarin 6; coumarin 545T; tris(8-quinolinolate)aluminum (abbreviation:Alq₃); 9,9′-biantryl; 9,10-diphenylanthracene (abbreviation: DPA);9,10-bis(2-naphthyl)anthracene (abbreviation: DNA); and the like. Also,another substances can be employed.

Further, triplet excited light emitting substances including metalcomplexes and the like may be used for a light emitting layer inaddition to the foregoing singlet excited light emitting substances. Forexample, pixels emitting red light in which half-life period of theluminance is relatively shorter than pixels emitting green and bluelights are formed by a triplet excited light emitting substance, and thepixels emitting green and blue lights are formed by singlet excitedlight emitting substances. Since the triplet excited light emittingmaterial has an excellent light emitting efficiency, it has a feature ofrequiring low power consumption in order to obtain a same level ofluminance as compared with the singlet excited light emittingsubstances. That is, when the pixels for emitting red light are formedby the triplet excited light emitting material, a small amount ofcurrent flowing through the light emitting element is required, therebyimproving the reliability. To reduce power consumption, in turn, pixelsemitting red and green lights may be formed of the triplet excited lightemitting materials, while pixels emitting blue light may be formed of asinglet excited light emitting material. In the case where lightemitting elements that emit green lights, which has high visibility withrespect to human eyes, are also formed of the triplet excited lightemitting material, power consumption can be further reduced.

As an example of the triplet excited light emitting substances, there isone that uses a metal complex as a dopant. In particular, a metalcomplex with platinum, which is a third transition element, as itscentral metal, a metal complex with iridium as its central metal and thelike are known. The triplet excited light emitting substances are notlimited to these compounds, and it is possible to employ a compoundhaving an above-mentioned structure and including an element thatbelongs to groups 8 to 10 of the periodic table as its central metal.

With respect to substances with excellent electron transportingproperties among substances with superior carrier transportingproperties, for example, metal complexes having quinoline skeleton orbenzoquinoline skeleton such as tris(8-quinolinolate)aluminum(abbreviation: Alq₃); tris(5-methyl-8-quinolinolate)aluminum(abbreviation: Almq₃); bis(10-hydroxybenzo[h]quinolinato)beryllium(abbreviation: BeBq₂); andbis(2-methyl-8-quinolinolate)-4-phenylphenolate-aluminum (abbreviation:BAlq) can be given. As substances having superior hole transportingproperties, for example, the following substances can be cited: aromaticamine (i.e., one having a benzene ring-nitrogen bond) based compoundssuch as 4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]-biphenyl (abbreviation:α-NPD); 4,4′-bis[N-(3-methylphenyl)-N-phenyl-amino]-biphenyl(abbreviation: TPD); 4,4′, 4″-tris(N,N-diphenyl-amino)-triphenylamine(abbreviation: TDATA); and 4,4′,4″-tris[N-(3-methylphenyl)-N-phenyl-amino]-triphenylam ine(abbreviation: MTDATA). With respect to substances having extremelysuperior electron injecting properties among the substances withexcellent carrier injecting properties, compounds of alkali metal oralkali earth metal such as lithium fluoride (LiF), cesium fluoride (CsF)and calcium fluoride (CaF₂) can be cited. In addition, a mixture of asubstance having a high electron transportation property such as Alq₃and alkali earth metal such as magnesium (Mg) may be used. With respectto substances having the superior hole injecting properties, forexample, metal oxide such as molybdenum oxide (MoOx), vanadium oxide(VOx), ruthenium oxide (RuOx), tungsten oxide (WOx) and manganese oxide(MnOx) can be cited. Besides, phthalocyanine based compounds such asphthalocyanine (abbreviation: H₂Pc) and copper phthalocyanine (CuPC) canbe mentioned. Further, a high molecular weight material mixingpolystyrenesulfonic acid (PSS) that is a substance with the excellenthole injecting/transporting properties and poly(ethylene dioxythiophene)(PEDOT) and the like can be used.

As compared with the low molecular weight organic light emittingmaterials, the high molecular weight organic light emitting materialshave higher physical strength, which results in more durable elements.In addition, since the high molecular weight organic light emittingmaterials can be formed by application, an element can be formedrelatively easily.

As specific examples of a semiconductor layer containing a crystallinecomponent that can be used as the second semiconductor layers 103 or thesecond semiconductor layers 203, single crystalline/polycrystallinesilicon, silicon germanium and the like can be cited. These substancesmay be formed by laser crystallization. For example, they may be formedby crystallization with solid phase growth using nickel etc. Also, theymay be formed by both laser crystallization and solid phase growth. Inaddition, a semiamorphous semiconductor can be used as the secondsemiconductor layers 103 and 203.

The semiamorphous semiconductor has an intermediate structure between anamorphous structure and a crystalline structure (also including a singlecrystalline structure and a polycrystalline structure), and a thirdcondition that is stable in term of free energy, and further includes acrystalline region having a short range order along with latticedistortion. A crystal region with a size of 0.5 to 20 nm can be observedin at least a part of the semiamorphous semiconductor film. Ramanspectrum originated L-O phonon is shifted toward lower wavenumbers than520 cm⁻¹. The diffraction peaks of (111) and (220), which are believedto be derived from Si crystal lattice, are observed in the semiamorphoussemiconductor by X-ray diffraction. The semiamorphous semiconductorcontains hydrogen or halogen of at least 1 atom % or more as aneutralizing agent for dangling bonds. The semiamorphous semiconductoris also referred to as a microcrystalline semiconductor. Thesemiamorphous semiconductor is formed by glow discharge decompositionwith silicide gas (plasma CVD). As for the silicide gas, SiH₄, Si₂H₆,SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄, and the like can be used. The silicide gasmay also be diluted with H₂, or a mixture of H₂ and one or more of raregas elements selected from He, Ar, Kr and Ne. The dilution ratio is setto be in the range of 1:2 to 1:1,000. The pressure is set to beapproximately in the range of 0.1 to 133 Pa. The power frequency is setto be 1 to 120 MHz, preferably, 13 to 60 MHz. The substrate heatingtemperature may be set to be 300° C. or less, preferably, 100 to 250° C.With respect to impurity elements contained in the film, eachconcentration of impurities for atmospheric constituents such as oxygen,nitrogen and carbon is preferably set to be 1×10²⁰ atoms/cm³ or less. Inparticular, the oxygen concentration is set to be 5×10¹⁹ atoms/cm³ orless, preferably, 1×10¹⁹ atoms/cm³ or less. The mobility of a TFT (thinfilm transistor) using the semiamorphous semiconductor is approximately1 to 10 cm²/Vsec.

In the light emitting elements 114 and 215, the electrodes 110 and 211of the light emitting elements may serve as anodes and the electrodes113 and 214 may serve as cathodes, respectively. Alternatively, theelectrodes 110 and 211 thereof may serve as cathodes while theelectrodes 113 and 214 may serve as anodes. In the former case, thetransistors connected to the respective light emitting elements arep-channel transistors. Or, in the latter case, the transistors connectedto the respective light emitting elements are n-channel transistors.

In the pixel portions of the light emitting devices according to thepresent invention, plural pixels including the above-mentioned lightemitting elements 114 and 215, and transistors for driving the lightemitting elements are arranged in matrix form. Note that light emittinglayers having different light-emitting wavelength bands may be formed ineach pixel so as to perform color display. Typically, light emittinglayers corresponding to respective colors of R (red), G (green) and B(blue) are formed. In this case, when a filter (a colored layer) thattransmits lights of the wavelength bands is provided at a side of thelight emitting device through which light generated from the lightemitting layer is emitted, color purity can be improved and specular reflexion (reflection) of a pixel portion can be prevented. By providingthe filter (colored layer), a circular polarizing plat etc., which hasconventionally been required, can be eliminated. Therefore, loss oflight emitted from the light emitting layers can be reduced.Furthermore, change in color tone, that is caused in the case where apixel portion (a display screen) is seen obliquely, can be furtherreduced.

A configuration with a light emitting layer capable of emittingmonochromatic light, white light for example, can be achieved instead ofperforming a color display by providing the light emitting layerscorresponding to respective colors. When using a white light emittingmaterial, a color display can be achieved by providing filters (coloredlayers) that transmits light of a certain wavelength toward a lightemitting direction of a pixel.

In order to form a light emitting layer that emits white light, forexample, white light emission can be obtained by sequentially laminatingAlq₃, Alq₃ partially doped with Nile red, p-EtTAZ and TPD (aromaticdiamine) by using vapor deposition. Also, when a light emitting layer isformed by applying a liquid using spin coating, the light emitting layeris preferably baked by vacuum heating after application. For example, anaqueous solution of poly(ethylene dioxythiophene)/poly(styrenesulfonicacid) (PEDOT/PSS) may be applied over the entire surface of thesubstrate and baked. Afterwards, a solution of polyvinyl carbazole (PVK)doped with a pigment (such as 1,1,4,4-tetraphenyl-1,3-butadiene (TPB),4-dicyanomethylene-2-methyl-6-(p-dimethylamino-styryl)-4H-p yran (DCM1),Nile red and coumarin 6) may then be applied over the entire surfacethereof and baked.

A light emitting layer may be formed to include a single layer insteadof the above-described light emitting layers including the plurallayers. In this case, 1,3,4-oxadiazole derivative (PBD) may be dispersedin polyvinyl carbazole (PVK). In addition, white light emission can beobtained by dispersing 30 wt % PBD and dispersing a suitable amount offour kinds of pigments (TPB, coumarin 6, DCM1 and Nile red).

When the light emitting element, which is a constituent element of thelight emitting device according to the present invention, is appliedwith a forward bias voltage, it can emit light. Pixels of a displaydevice formed by using a light emitting element can be driven by anactive matrix method. In either case, each pixel emits light by beingapplied with a forward bias voltage at a certain timing; however eachpixel does not emit light in a certain period. In the non-light-emittingperiod, a reverse bias voltage is applied to the light emitting elementso that the reliability of the light emitting element can be improved.The light emitting element has deterioration modes of reducing lightintensity under a certain drive condition or reducing luminanceapparently due to expansion of a non-light-emitting region within eachpixel. When the light emitting element is driven by AC drive such thateach pixel is applied with a forward bias voltage and a reverse biasvoltage alternately, the deteriorations of the light emitting elementcan be hindered, thereby increasing the reliability of the lightemitting device.

Note that the above-described configurations may be applicable to lightemitting devices of another embodiment modes besides the light emittingdevices as shown in FIGS. 3C and 5C.

[Embodiment Mode 4]

In light emitting devices of the invention, circuits provided in pixelportions for driving light emitting elements will be described in thepresent embodiment mode. Note that a circuit for driving a lightemitting device is not limited to ones described in the presentembodiment mode.

As shown in FIG. 6A, a light emitting element 301 is connected to acircuit for driving each light emitting element. The circuit includes adriving transistor 321 for determining light-emission/non-light-emissionof the light emitting element 301 by an image signal, a switchingtransistor 322 for controlling input of the image signal and an erasingtransistor 323 for making the light emitting element 301 anon-light-emitting state regardless of the image signal. In this case, asource (or a drain) of the switching transistor 322 is connected to asource signal line 331. Sources of the driving transistor 321 and theerasing transistor 323 are connected to a power supply line 332extending in parallel with the source signal line 331. A gate of theswitching transistor 322 is connected to a first scanning line 333. Agate of the erasing transistor 323 extending in parallel with the firstscanning line 333 is connected to a second scanning line 334. Further,the driving transistor 321 and the light emitting element 301 areconnected in series with each other.

A driving method for making the light emitting element 301 emit lightwill be described. When the first scanning line 333 is selected in awriting period, the switching transistor 322 with the gate connected tothe first scanning line 333 is turned on. When the image signal input inthe source signal line 331 is input in the gate of the drivingtransistor 321 via the switching transistor 322, current flows throughthe light emitting element 301 from the power supply line 332, andhence, the light emitting element emits light. At this moment, theluminance of light emission is determined depending on the amount ofcurrent flowing through the light emitting element 301.

FIG. 7 is a top view of a pixel portion for a light emitting devicehaving the circuit as shown in FIG. 6A. Reference numerals as shown inFIG. 7 represent same portions in FIG. 6A, respectively. Also, anelectrode 84 of the light emitting element 301 is shown in FIG. 7.

The configurations of circuits connected to respective light emittingelements are not limited to the above-mentioned configuration. Forexample, the after-mentioned configurations as shown in FIGS. 6B and 6Cmay also be used.

Next, the circuit as shown in FIG. 6B will be described. As shown inFIG. 6B, the circuit for driving each light emitting element isconnected to a light emitting element 801. The circuit includes adriving transistor 821 for determining light-emission/non-light-emissionof the light emitting element 801 by an image signal, a switchingtransistor 822 for controlling input of the image signal, an erasingtransistor 823 for making the light emitting element 801 anon-light-emitting state regardless of the image signal, and a currentcontrolling transistor 824 for controlling the amount of current flowingthrough the light emitting element 801. In this case, a source (or adrain) of the switching transistor 822 is connected to a source signalline 831. Sources of the driving transistor 821 and the erasingtransistor 823 are connected to a power supply line 832 extending inparallel with the source signal line 831. A gate of the switchingtransistor 822 is connected to a first scanning line 833. A gate of theerasing transistor 823 is connected to a second scanning line 834extending in parallel with the first scanning line 833. Further, thedriving transistor 821 and the light emitting element 801 are connectedin series with each other while sandwiching the current controllingtransistor 824 therebetween. A gate of the current controllingtransistor 824 is connected to the power supply line 835. The currentcontrolling transistor 824 is formed and controlled such that currentflows in a saturation region of voltage-current (Vd-Id) characteristics.This can determine the amount of current flowing through the currentcontrolling transistor 824.

A driving method for making the light emitting element 801 emit lightwill be described. When the first scanning line 833 is selected in awriting period, the switching transistor 822 with the gate connected tothe first scanning line 833 is turned on. The image signal input in thesource signal line 831 is input in the gate of the driving transistor821 via the switching transistor 822. Current flows through the lightemitting element 801 from the power supply line 832 via the drivingtransistor 821 and the current controlling transistor 824 that is turnedon upon receiving the signal from a power wiring 835, and hence, thelight emitting element emits light. At this moment, the amount ofcurrent flowing through the light emitting element 801 is determineddepending on the current controlling transistor 824.

FIG. 8 is a top view of the pixel portion for the light emitting devicehaving the circuit as shown in FIG. 6B. Reference numerals in FIG. 8represent the same portions of FIG. 6B. Note that an electrode 94 of thelight emitting element 801 is depicted in FIG. 8, though the lightemitting element 801 is not illustrated therein.

Next, the circuit as shown in FIG. 6C will be described. The circuit fordriving each light emitting element is connected to alight emittingelement 401. The circuit includes a driving transistor 421 fordetermining light-emission/non-light-emission of the light emittingelement 401 by an image signal and a switching transistor 422 forcontrolling input of the image signal. In this case, a source (or adrain) of the switching transistor 422 is connected to a source signalline 431. A source of the driving transistor 421 is connected to a powersupply line 432 extending in parallel with the source signal line 431. Agate of the switching transistor 422 is connected to a scanning line433. Further, the driving transistor 421 and the light emitting element401 are connected in series with each other.

A driving method for making the light emitting element 401 emit lightwill be described. When the scanning line 433 is selected in a writingperiod, the switching transistor 422 with the gate connected to thescanning line 433 is turned on. When the image signal input in thesource signal line 431 is input in the gate of the driving transistor421 via the switching transistor 422, current flows through the lightemitting element 401 from the power supply line 432, and hence, thelight emitting element emits light. At this moment, the luminance oflight emission is determined depending on the amount of current flowingthrough the light emitting element 401.

[Embodiment Mode 5]

FIG. 9 is a top view of a light emitting device, wherein the lightemitting device manufactured according to the present invention issealed and mounted with an external connection terminal.

A first substrate 1001 and a second substrate 1021 are attached to eachother and overlapped with each other. A pixel portion 1011, a drivercircuit portion 1012 for driving a first scanning line, a driver circuitportion 1013 for driving a second scanning line, a driver circuitportion 1014 for driving a source signal line, and a connection wiringgroup 1015 (which is surrounded by a dashed line) are provided over thefirst substrate 1001. A shift register, a buffer, a switch and the likeare provided in the driver circuit portions 1012, 1013 and 1014. Theconnection wiring group 1015 and an FPC (a flexible printed circuit)1031 that is an external connection terminal are connected to each otherwith an anisotropic conductive adhesive agent. Plural pixels each ofwhich includes a light emitting element and a circuit for driving thelight emitting element are aligned in the pixel portion 1011. Signalssuch as video signals, clock signals, start signals and reset signalsare sent to the driver circuit portions 1012, 1013 and 1014, a powersupply line 1016 and the like from a controller via the FPC 1031.Further, the signals are sent to the pixel portion from the drivercircuit portions 1012, 1013 and 1014, and the power supply line 1016.

Note that the driver circuit portions are not necessary to be providedover the same substrate as the pixel portion 1011 as mentioned above.For example, these driver circuit portions may be provided outside ofthe substrate by using an FPC over which a wiring pattern is formed andan IC chip is mounted thereover (TCP).

The foregoing light emitting device is manufactured according to themanufacturing method capable of reducing defects due to electrostaticcharge and utilizing a substrate surface effectively. That is, plurallight emitting devices are manufactured over one substrate all together,which results in low manufacturing cost.

An embodiment of an electronic appliance mounted with a light emittingdevice according to the present invention is shown in FIG. 10. FIG. 10shows a cellular phone manufactured according to the present invention,which includes a main body 5552, a display portion 5551, an audio outputportion 5554, an audio input portion 5555, operational switches 5556,5557, an antenna 5553 and the like. By incorporating the light emittingdevice of the invention as a display portion, display whose defects dueto electrostatic charge that is caused during manufacturing process canbe reduced is achieved, and hence, a cellular phone capable ofdisplaying a favorable image can be manufactured. In addition to thecellular phone, the display device according to the present inventioncan be applied to a digital camera, a car navigation system, a displaydevice and the like. This can reduce the display defects due to theelectrostatic charge caused during the manufacturing process.Consequently, the digital camera, the car navigation system, the displaydevice and the like each of which can provide favorable images can becompleted.

As set forth above, the light emitting devices of the invention aresuitable as display portions for various kinds of electronic appliances.

1. A method for manufacturing a light emitting device, comprising a stepof forming a top-gate type transistor for driving a light emittingelement, wherein a first grid-like semiconductor layer extending in rowsand columns is formed over a substrate and a plurality of secondisland-like semiconductor layers are formed between the first grid-likesemiconductor layer.
 2. The method for manufacturing the light emittingdevice according to claim 1, wherein the method further comprises a stepof adding an impurity to the first grid-like semiconductor layer and theplurality of second island-like semiconductor layers.
 3. A method formanufacturing a light emitting device, comprising a step of forming atop-gate type thin film transistor for driving a light emitting element,wherein a first grid-like semiconductor layer extending in rows andcolumns is formed over a substrate and a plurality of second island-likesemiconductor layers are formed between the first grid-likesemiconductor layer.
 4. The method for manufacturing the light emittingdevice according to claim 3, wherein the method further comprises a stepof adding an impurity to the first grid-like semiconductor layer and theplurality of second island-like semiconductor layers.
 5. A method formanufacturing a light emitting device, comprising a step of forming atransistor for driving a light emitting element in which the transistoris formed by sequentially laminating an island-like semiconductor layer,an insulating layer and a conductive layer, wherein a first grid-likesemiconductor layer extending in rows and columns is formed over asubstrate, and a plurality of second island-like semiconductor layersincluding the island-like semiconductor layer are formed between thefirst grid-like semiconductor layer.
 6. The method for manufacturing thelight emitting device according to claim 5, wherein the method furthercomprises a step of adding an impurity to the first grid-likesemiconductor layer and the plurality of second island-likesemiconductor layers.